This invention relates to dielectric resonator apparatus and methods of adjusting their characteristics. More particularly, this invention relates to dielectric resonator apparatus having one or more coaxial dielectric resonators and methods of adjusting characteristics of such apparatus.
Prior art technology in the field of dielectric resonator apparatus will be described first with reference to FIGS. 10 and 11 showing a conventional dielectric resonator adapted to function as a three-stage bandpass filter, comprising an approximately parallelopipedic block 100 made of a dielectric material, molded resin members 110 made of a resin material, metallic pins 120 and upper and lower metallic casing members 130 and 140 for magnetic shielding. The dielectric block 100 has three resonator-forming throughholes 102a, 102b and 102c therethrough and coupling throughholes 103a and 103b respectively between the resonator-forming throughholes 102a and 102b and between the throughholes 102b and 102c. These throughholes 102a, 102b, 102c, 103a and 103b penetrate the dielectric block 100 between its first end surface 101a and second end surface 101b, having openings thereon. An inner conductor 104 is formed on the inner surface of each of the resonator-forming throughholes 102a, 102b and 102c, with one end extending to one of the openings and the other end extending to the other of the openings. An outer conductor 105 is formed on the outer surfaces of the dielectric block 100 except the second end surface 101b such that the inner conductors 104 are each connected with the outer conductor 105 (at the shorted end parts) on the first end surface 101a but insulated from the outer conductor 105 (at the open end parts) on the second end surface 101b.
The molded resin members 110 of a dielectric resin material are each formed by molding with an input/output terminal 111 inserted by an insert-molding process and also with a pin-accepting hole 112 for having a metallic pin 120 inserted therein. These molded resin members 110 are inserted into the resonator-forming throughholes 102a and 102c, and the metallic pins 120 are inserted into their pin-accepting holes 112 so as to be in electrically conductive relationship with the input/output terminals 111 and to provide external coupling capacitance C.sub.e with the inner conductors 104. The lower metallic casing member 140 is provided with a plurality (five, as shown in FIG. 11) of tab terminals 141 for grounding and fastening pieces 142 for fastening the dielectric block 100 therein. The tab terminals 141 are adapted to be soldered onto a grounding terminal of a circuit board (not shown) with a desired circuitry formed thereon. The upper and lower metallic casing members 130 and 140 are also provided with holes 133 and protrusions 143, respectively, such that they can be engaged together. Although the second end surface 101b of the dielectric block 100 is not covered with the outer conductor 105, electromagnetic waves are prevented from leaking outside therefrom because it is shielded by the mutually engaged upper and lower metallic casing members 130 and 140 as shown in FIG. 10.
The three coaxial dielectric resonators thus formed inside the single dielectric block 100 are magnetically coupled through the coupling throughholes 103a and 103b. The degree of coupling between the coaxial dielectric resonators can be adjusted by varying conditions such as the diameters, lengths and the positions of the coupling throughholes 103a and 103b. The coaxial dielectric resonators formed in the resonator-forming throughholes 102a and 102c are also connected individually with the input/output terminals 111 through the external coupling capacitance C.sub.e and the metallic pins 120. The dielectric resonator apparatus thus formed can function as a three-stage bandpass filter. The level of each external coupling capacitance C.sub.e can be adjusted by varying the distance by which the metallic pins 120 are inserted into the pin-accepting holes 112.
Dielectric resonator apparatus as shown in FIGS. 10 and 11 are not compact because the upper and lower metallic casing members 130 and 140 must be provided and the tab terminals 141 protrude from them, and the number of parts is also large. In other words, such a dielectric resonator apparatus is not suited for surface-mounting and has the disadvantage of being costly.
In view of such problems as discussed above, there has also been proposed another kind of dielectric resonator apparatus as shown in FIGS. 12 and 13. The exemplary apparatus shown in FIGS. 12 and 13 is designed to function as a two-stage bandpass filter, comprising a block 201 of an approximately rectangular parallelopiped made of a dielectric material, having two resonator-forming throughholes 202a and 202b which penetrate the dielectric block 201 between its first end surface 201a and second end surface 201b, having openings thereon. An inner conductor 204 is formed on the inner surface of each of the resonator-forming throughholes 202a and 202b, with one end extending to one of the openings (on the first end surface 201a) and the other end extending towards but not reaching the other of the openings (on the second end surface 201b). An outer conductor 205 is formed on the outer surfaces of the dielectric block 201 inclusive of the second end surface 201b such that the inner conductors 204 are each connected with the outer conductor 205 (at the shorted end parts) on the first end surface 201a but insulated from the outer conductor 205 (at the open end parts) on the second end surface 201b. In other words, each of the resonator-forming throughholes 202a and 202b has an insulating section 208 which separates the inner conductor 204 from the outer conductor 205 and at which there is no conductor present. On the outer peripheral surface of the dielectric block 201, there are input/output electrodes 206 and 207 formed, insulated from the outer conductor 205. The outer conductor 205 is adapted to be soldered onto a grounding terminal of a circuit board (not shown).
The two coaxial dielectric resonators thus formed inside the single dielectric block 201 are capacitively coupled across the two insulating sections 208. External coupling capacitance C.sub.e also appears between electrode 206 and one of the inner conductors 204 and between electrode 207 and the other inner conductor 204 such that the coaxial dielectric resonators are each connected to the input/output electrode 206 or 207 through the external coupling capacitance C.sub.e. The dielectric resonator apparatus thus formed can function as a two-stage bandpass filter. The level of each external coupling capacitance C.sub.e can be adjusted by varying conditions such as the areas and positions of the input/output electrodes 206 and 207 and the diameters of the resonator-forming throughholes 202a and 202b.
With a dielectric resonator apparatus as explained above with reference to FIGS. 12 and 13, however, the areas and positions of the input/output electrodes 206 and 207 must be redesigned whenever an attempt is made to change the external coupling capacitance C.sub.e in order to adjust or change functional characteristics of the apparatus such as its frequency characteristics. This means that it takes a long time to design such a dielectric resonator apparatus properly.